Pluto Gets the BootSolar System Shrinks to 8 PlanetsBy A.J.S. Rayl

Pluto got the boot today
when 2,500 astronomers from 75 countries at the International
Astronomical Union (IAU) annual meeting in Prague voted on a resolution
that aims to define exactly what a planet is.

What this means is that Pluto
has been ejected from the planetary brotherhood. It is no longer
considered a planet, but a dwarf planet. As a result, the solar system
shrunk today by 1 planet, meaning that the solar system now consists of
8 recognized “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus and Neptune.

In short, a “planet” is now defined as
a celestial body that (a) is in orbit around the Sun, (b) has
sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape, and (c)
has cleared the neighborhood around its orbit.

This marks
the first time the IAU has put forth scientific criteria for a planet
and voted on it. A new distinct class of objects called “dwarf planets”
was also defined as the astronomers agreed that “planets” and “dwarf
planets” are two distinct classes of objects.

A dwarf planet, according to the new definition, is a celestial body
that (a) is in orbit around the Sun, (b) has sufficient mass for its
self-gravity to overcome rigid body forces so that it assumes a
hydrostatic equilibrium (nearly round) shape, (c) has not cleared the
neighborhood around its orbit, and (d) is not a satellite.

All other objects orbiting the Sun will now referred to collectively as “Small Solar System Bodies."

The first members of the “dwarf planet” category are Ceres, Pluto,
and 2003 UB313 (aka Xena). More “dwarf planets” are expected to be
announced by the IAU in the coming months and years. Currently, about a
dozen candidate “dwarf planets” are listed on IAU’s “dwarf planet”
watchlist, which keeps changing as new objects are found and the
physics of the existing candidates becomes better known.

For Pluto supporters, not all is lost. The “dwarf planet” Pluto
is being recognized, according to the IAU, as "an important proto-type
of a new class of trans-Neptunian objects." The IAU will set up a
process to name these objects.

"The classification doesn't
matter," said Louis Friedman, executive director of The Planetary
Society. "Pluto -- and all solar system objects -- are mysterious and
exciting new worlds that need to be explored and better understood.
Anytime we visit a new world -- planet, moon, asteroid, comet, whatever
-- we make exciting and surprising new discoveries about the evolution
of our solar system and about our own planet."

"Being The
Planetary' Society, however, we do appreciate the intense interest in
the question -- what is a planet -- by both professionals and the
general public, and we want to hear from them," Friedman added. The
Society is conducting a public poll on the web at http://planetary.org/explore/topics/pluto/poll.html.
In addition, the organization is also inviting people from around the
world to submit photos for a time capsule to be opened when the New
Horizons spacecraft reaches Pluto in 2015 as a way of connecting the
long term pursuit of space exploration with the rapid pace of change
here on Earth.

As
it turns out, the proposed resolution that made so much notice when it
was presented to the public last week -- and which would have expanded
the solar system to 12 planets -- passed, but it was a different draft
of the document in that it added the third criteria of a planet having
to clear its neighborhood around its orbit. Pluto's orbit crosses that
of Neptune, which is much larger, so the former 9th planet didn't meet
all the criteria.

A specially selected committee spent 2 years, under the auspices of the IAU, working on the proposal
to define what, exactly, should be deemed a planet, as well as a way to
better describe the difference between planets and smaller solar system
bodies, such as comets and asteroids.

By that committee's original draft of the resolution,
Ceres, the largest asteroid, would have become known as a "dwarf
planet," while Pluto, Charon, and 2003 UB313/Xena would form a new
class of objects called "plutons," and the term "minor planet" would
have faded from astronomy's lexicon. Objects not classified as planets
would be referred to as "small solar system bodies." Under that draft definition, Pluto, as a pluton, though distinct from the solar system's 8 "classical planets," would still have been a planet.

When
the committee released the proposed resolution last week, the "early
returns," according to panel member Richard Binzel, professor of
planetary science at the Massachusetts Institute of Technology (MIT),
were looking good. But as the week progressed, the debates got more and
more heated, and the resolution was defeated in a straw poll of
planetary scientists during the first major debate on the plan last
Friday. That sent rumors flying around the world that the resolution
would go down in defeat. Another vote, just among planetary scientists
on Tuesday, ended in a draw.

Geologists were criticizing the
panel for failing to consider the traditional use of the word "pluton"
as the name of an igneous rock. "The committee was aware of the
geological term, but felt 'pluton' could be shared by the two
communities, with qualifiers used in cases where there might be
confusion -- such as 'planetary pluton' and "igneous 'pluton,'" panel
member Dava Sobel, author and historian, told The Planetary Society.

Meanwhile,
various planetary scientists had numerous notions and so the original
draft of the proposed resolution was modified a couple of times, before
it was voted on today.

For as many who are satisfied, there
are others who are disappointed. Even before the vote, Sobel expressed
"surprise" at "the reluctance of some people to accept 12 planets or
more," noting that she "embrace(d) the possibility of a wider solar
system, especially since the solar system seems to hold more than
anyone ever imagined."

As it turns out, in the big picture, the
new definition may mean little since the IAU notes that it will only
apply to planets in our solar system. That leaves a universe of
spherical objects out there for astronomers to ponder.

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